Look for 32-bit libraries in /usr/lib32 for MIPS O32 on FreeBSD.

Summary:
FreeBSD N64 MIPS systems can include 32-bit libraries for O32 in
/usr/lib32 similar to the 32-bit compatibility libraries provided
for FreeBSD/amd64 and FreeBSD/powerpc64.

Reviewers: dim

Reviewed By: dim

Differential Revision: https://reviews.llvm.org/D42972

llvm-svn: 324948